PRECONDITIONING WITH BRIGHT LIGHT EVOKES A PROTECTIVE RESPONSE AGAINST LIGHT DAMAGE IN THE RAT RETINA

Constant exposure to bright light induces photoreceptor degeneration a nd at the same time upregulates the expression of several neurotrophic factors in the retina, At issue is whether the induced neurotrophic f actors protect photoreceptors, We used a preconditioning paradigm to s how that animals preconditioned with bright light became resistant to subsequent light damage. The preconditioning consisted of a 12-48 hr p reexposure, followed by a 48 hr ''rest phase'' of normal cyclic lighti ng. The greatest protection was achieved by a 12 hr preexposure, Preco nditioning induces a prolonged increase in two endogenous neurotrophic factors: basic fibroblast growth factor (bFGF) and ciliary neurotroph ic factor (CNTF). It also stimulates the phosphorylation of extracellu lar signal-regulated protein kinases (Erks) in both photoreceptors and Muller cells, These findings indicate that exposure to bright light i nitiates two opposing processes: a fast degenerative process that kill s photoreceptors and a relatively slower process that leads to the pro tection of photoreceptors, The extent of light damage, therefore, depe nds on the interaction of the two processes, These results also sugges t a role of endogenous bFGF and CNTF in photoreceptor protection and t he importance of Erk activation in photoreceptor survival.